Maintenance work of exchanging lubrication oil of an engine of an automobile or the like at proper timing is very important for prolonging the service life of the engine. In general, an oil gauge, which is inserted into an oil pan, is removed periodically in order to visually check the degree of contamination of the oil, whereby the person making the check can grasp the degree of deterioration of the oil. However, a driver who drives an automobile under the pressure of necessity but has no interest in the automobile, or a driver who is not mechanically inclined, generally first becomes aware of contamination of oil when a conscientious gas station attendant checks the oil. Unbelievably, some drivers change oil only at the time of automobile inspection. Although many automobiles have an oil warning indicator on their dashboard panels, the indictor merely reports the result of detection of oil level and does not report the degree of contamination.
In view of the foregoing, in Japanese Kohyo (PCT) Patent Publication No. 11-507434, Japanese Patent Application Laid-Open (kokai) No. 5-264495, Japanese Patent Application Laid-Open (kokai) No. 63-168549, and Japanese Patent Application Laid-Open (kokai) No. 59-102151, there are proposed apparatuses for detecting the degree of deterioration of engine oil by measuring the electrostatic capacitance of the oil or the dielectric constant thereof, which has a correlation with the electrostatic capacitance. One cause of oil deterioration is an increase in total acid value. This is considered to occur because base oil, which is the predominant component of the engine oil, oxidizes, with a resultant increase in the quantity of carboxylic acid and alcohol, which are polar substances. As a result, the dielectric constant of the oil increases, and the electrostatic capacitance of the oil increases accordingly. Therefore, the degree of deterioration of the oil can be known through detection of the increased electrostatic capacitance.
Incidentally, performing the above-described measurement of the electrostatic capacitance of oil requires an electric circuit for measurement purpose. The electric circuit has a pair of electrodes which are dipped in oil in a mutually facing condition and which serve as a capacitance measurement sensor. The electrodes constitute a sensing capacitor, in which oil serves as a dielectric. Basically, the capacitance of this capacitor is determined through measurement of the AC impedance of the capacitor, which measurement requires provision of peripheral circuits, including a signal generation circuit for measurement, an output processing circuit for processing a response wave signal, and other circuits. These peripheral circuits and the sensing capacitor are connected together by use of a cable such as a pair of twisted wires or a coaxial cable in order to prevent entry of noise. However, when the length of the cable is increased, the influence of parasitic capacitance of the cable itself increases, thereby preventing accurate measurement of electrostatic capacitance of oil. Therefore, for accurate measurement of electrostatic capacitance of oil, the length of the cable for connecting the sensing capacitor and the peripheral circuits must be shortened to the extent possible. As a result, the peripheral circuits must be disposed in the vicinity of the engine, to which the oil pan is attached.
In the vicinity of the operating engine, the temperature unavoidably increases even though the engine is cooled by water, and the temperature changes within a relatively wide range from atmospheric temperature to about 130° C., depending on the operating conditions. Therefore, the temperature of the peripheral circuits for measurement disposed near then engine also changes accordingly. As a result, even when the degree of deterioration of oil is not changed, the level of the detection output changes if the temperature of the peripheral circuits used for measurement changes. Further, because of long-term exposure to such high temperature, the characteristics of the peripheral circuits deteriorate to some degree, whereby the level of the detection output changes to no small extent. Both the temperature change and the long-term deterioration of the circuit characteristic lead to the problem that accurate measurement of electrostatic capacitance becomes impossible.
An object of the present invention is to provide an oil deterioration detection apparatus which can always perform accurate detection of deterioration of oil on the basis of electrostatic capacitance of the oil, even when the temperature of a circuit system for measuring the electrostatic capacitance of the oil changes.